Development of an 'ice tsunami' a matter of probability: researcher

Hey there, time traveller!This article was published 14/5/2013 (1305 days ago), so information in it may no longer be current.

It sounds like the stuff of speculative fiction: A wall of ice materializes on a lakeshore within the space of minutes, toppling trees and smashing structures in its path.

But scientists say the ice that devastated Ochre Beach on Dauphin Lake last week was merely an uncommon phenomenon, capable of occurring on the shore of almost any large temperate body of water when sustained high winds send free-floating pieces of ice in motion.

PHIL HOSSACK / WINNIPEG FREE PRESS

Ice on Dauphin Lake in the community of Ochre Beach threatens to blow back onshore and do further damage to homes and cottages Monday. Purchase Photo Print

The phenomenon that damaged or destroyed 27 properties on the southern shore of Dauphin Lake on Friday is formally known as an ice shove. Engineers and geographers describe these events as unusual but hardly unknown.

'This is a fairly normal thing to happen. It's just a matter of timing. It's just getting the right combination of factors. It's all probability'

-- Klaus Hochheim, research associate at the U of M's Centre for Earth Observation Science

"This is a fairly normal thing to happen. It's just a matter of timing," said Klaus Hochheim, a research associate at the University of Manitoba's Centre for Earth Observation Science. "It's just getting the right combination of factors. It's all probability."

The first factor is the presence of ice floating on a large body of water that's in the process of thawing. While solid sheets won't move, any form of free-floating ice -- large blocks or smaller shards -- can be pushed around by wind.

When the wind is strong enough and blows long enough to push that ice in one direction, it can become an ice shove when it hits the shore. The leading edge of this mass of ice will stop, forcing the trailing ice to pile up on top of it.

Given enough momentum, the resulting pile can surge onto the shore.

"It's like a freight train, and you have all these cars at the back end with nowhere to go," said Jay Doering, a University of Manitoba civil engineering professor. "Some have even referred to it as an ice tsunami."

The reason ice shoves may be unfamiliar to Manitobans is this phenomenon likely occurs most often in unpopulated areas and therefore goes unreported. The most wind-exposed sections of Manitoba's largest lakes are largely undeveloped areas such as the beach ridges at Delta Marsh on Lake Manitoba and Netley Marsh on Lake Winnipeg, both of which bear scars from ice shoves.

"We know from previous shoreline damage that ice piling happens regularly," said Minnesota State geologist Harvey Thorleifson, a former Manitoban.

What was unusual about the event at Ochre Beach was the extreme amount of property damage. The final damage tally -- as well as the number of homes and cottages that will wind up being completely lost -- will not be known until the cleanup is completed, said Doug McNeil, deputy minister of Manitoba Infrastructure and Transportation.

The height of the ice pile on Dauphin Lake was also extreme, although ice piles as high as 12 metres have been documented. The low gradient on the shore of all of Manitoba's Great Lakes presents little resistance to the forward progress of ice piles.

While bridge piers are designed to withstand the pressure of moving ice, homes and cottages cannot.

"A wooden house is nothing," Doering said.

In theory, it could be possible to forecast ice shoves by placing transmitters on ice floes, similar to the ones U of M scientists use to monitor the movements of Arctic sea ice.

In practice, however, this would be difficult, Doering surmised.

"You'd need real-time information about the ice and then have to notice someone turned on the wind," he said.

Ice shoves also are not dependent on high water levels. Dauphin Lake stood at 856.1 feet above sea level over the weekend, which is 0.7 feet above the upper target level for the lake and almost five feet below the crest of the 2011 spring flood.

bartley.kives@freepress.mb.ca

Anatomy of an ice shove

On larger lakes and oceans, the ice-shove threat is restricted to the period when ice has started to melt but has not disappeared. Any form of free-floating ice, from large pans to medium-sized blocks to relatively small chunks, can contribute to an ice shove.

2. High winds

On a body of water with free-floating ice, sustained high winds can push pans, blocks or shards toward the shore. When the leading edge of this ice hits the shore, the friction will slow it down or stop it. But the absence of resistance on the water will allow the wind to continue shoving the ice toward the shore. Since the trailing ice can not move forward, it will pile up on to the leading ice. Given enough wind and ice, piles as high as 12 metres can materialize on shore in a matter of minutes.

3. Low gradient

While the gradient of the shore will not determine whether an ice shove occurs, a steeper slope will present more resistance to the forward progress of the ice pile. The gradient of the land alongside Manitoba's five Great Lakes -- Lake Winnipeg, Lake Manitoba, Lake Winnipegosis, Cedar Lake and Dauphin Lake -- is very low.

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